The present disclosure relates to a laser diode assembly and a semiconductor optical amplifier assembly.
Today, ultrashort-pulse ultrahigh-power laser is actively used for study of advanced scientific areas using a laser beam with a pulse time in attoseconds or femtoseconds. The ultrashort pulse laser is actively studied from scientific interest of elucidation of ultrafast phenomenon in picoseconds or femtoseconds, and besides, actively studied for practical use such as microfabrication or two-photon imaging using high peak power. A high-power ultrashort-pulse laser diode device including a GaN-based compound semiconductor with an emission wavelength band of a 405 nm is expected as a light source of a volumetric optical disk system expected as a next-generation optical disk system following the Blu-ray optical disk system, as a light source demanded in a medical or bioimaging field, or as a coherent light source covering the entire visible light range.
For example, titanium/sapphire laser is known as the ultrashort-pulse ultrahigh-power laser. However, the titanium/sapphire laser is an expensive, large solid-state laser light source, which is a main factor of disturbing spread of the laser technology. If the ultrashort-pulse ultrahigh-power laser may be achieved by laser diode or a laser diode device, size reduction, price reduction, low power consumption, and high stability are achieved, which is conceivably breakthrough in promoting spread of the laser technology in the above fields.
A laser diode assembly having an overall semiconductor configuration as such a high-peak-power picosecond-pulse light source in the 405 nm band typically has an MOPA (Master Oscillator and Power Amplifier) configuration. Specifically, the assembly is configured of laser diode generating a picosecond pulse, and a semiconductor optical amplifier (SOA) amplifying the generated picosecond pulse. Here, the optical amplifier directly amplifies a light signal without converting the light signal into an electrical signal, and has a laser structure without a resonator, and thus amplifies incident light with optical gain of the amplifier. A pulse light source generating a picosecond pulse with the MOPA configuration specifically includes a mode-locked laser diode assembly having an external resonator.
The mode-locked laser diode assembly is achieved by, for example, a multi-electrode mode-locked laser diode device and an external resonator disposed on an optical axis of the laser diode device. One end face of the multi-electrode mode-locked laser diode device often has a high-reflective coating layer thereon, and thus also serves as a mirror opposed to the external resonator. In addition, a wavelength selective element such as a diffraction grating or a bandpass filter including a dielectric multilayer film is disposed, making it possible to select an oscillation wavelength.
When the diffraction grating is used as the wavelength selective element, the external resonator is configured of the diffraction grating, and primary diffracted-light is returned to the laser diode device, thereby an oscillation wavelength may be selected. Such an arrangement is known as Littrow arrangement or Littman arrangement, and used for a continuous-oscillation tunable laser (for, example, see Japanese Unexamined Patent Application Publication No. 2001-284716, Heim et al, Electronics Letters, vol. 33, 16, p 1387 (1997), and Struckmeier et al, Optics Letters, vol. 24, 22, p 1573 (1999)). When a bandpass filter is used as the wavelength selective element, the bandpass filter is disposed between a laser diode device and an external resonator to provide wavelength selectivity (for example, see Japanese Unexamined Patent Application Publication No. 2002-164614).